Circuit protective variable ratio transformer system



Sept. 22, 1959 R. SHERMAN 2,905,864

CIRCUIT PROTECTIVE VARIABLE RATIO TRANSFORMER SYSTEM Original Filed Oct. 20, 1948 2 Sheets-Sheet 1 INVENTOR. RA LPH SHERMAN ATTQIPAEY Sept. 22, 1959 R. SHERMAN 2,905,854

CIRCUIT PROTECTIVE VARIABLE RATIO TRANSFORMER SYSTEM Original Filed Oct. 20, 1948 2 Sheets-Sheet 2 5 2 $1 67 7 6 as J 112 j f 713 13 523 4 68 g k 4 526 10 K536) RAZ PH SHERMAN misa 3 K; Fig. 4

ATTOJQAZE'Y United States Patent ()fifice 2,905,864 Patented Sept. 22, 1959- CIRCUIT PROTECTIVE VARIABLE RATIO TRANSFORNIER SYSTEM Ralph Sherman, Warren, Ohio; Alex Sherman and Arnold Sherman, executors of Ralph Sherman, deceased Original application October 20, 1948, Serial No. 55,605. Divit6led and this application April 23, 1953, Serial No. 351, 61

4 Claims. (Cl. 317-9) This application is a division of my co-pending application, Serial Number 55,605, now Patent No. 2,683,820, which in turn, is a continuation of my application, Serial #495,311 filed July 19, 1943 on which Patent No. 2,459,186 was granted January 18, 1949.

This invention relates to protection of means for permanently or intermittently supervising, testing, indicating and controlling the condition of contact between electrically conductive parts which are required to maintain intimate contact between them.

Certain features disclosed herein but not claimed are claimed in my aforesaid co-pending applications or in my co-pending application, Serial Number 719,367 filed December 31, 1946, now Patent No. 2,701,965.

An object of the invention is to insure reliability of apparatus for supervising or indicating contact condition by protection thereof against overloads of over-voltage and in this manner to render more reliable and safer electrical apparatus in which poor contact conditions may exist or arise.

Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

As is well known to those skilled in the art, electrical conditions in contacts or other parts of electrical circuits, and more especially in heavy current circuits, vary in dependency on the influences exerted on them by the media in which they are operating. The oxygen or the moisture in the air will cause the formation of an oxide layer on a contact surface. Dust, oil or grease may form insulating coatings thereon. Oxidation may also occur through the action of heat. Loosening of mechanical connections may occur through many causes. In all such cases there results a rise of resistance to the passage of current.

In the apparatus described in my parent application the variations in this intermediate resistance are rendered visible or audible in indicating or alarm devices, or are utilized to activate switches, permanently connected to the contacting parts to be protected or supervised, while these parts are traversed by the working current. In any case the attendants or operators are thus warned and induced to remedy the defect or, if necessary, to temporarily throttle or cut out the current.

Throughout the drawings afiixed to this specification and forming a part thereof, similar numerals are intended to designate similar parts. In the drawings:

Fig. 1 is a diagram of an alternating current system comprising an electrical melting furnace with an electrode mounted in a holder, in combination with means for continuously supervising, indicating and controlling the variations of contact between these parts during operation, making use of the working current for effecting this indication and control.

Fig. 2 illustrates the adaptation of the controlling system to lever switches and more particularly to a doublepole direct-current breaker, in which the supervising device also comprises an auxiliary switch.

Fig. 3 is a diagram of a system comprising a high terision current oil circuit-breaker in combination with a supervising and controlling device which comprises a similar auxiliary switch.

Fig. 4 is a longitudinal sectional view of the core of the transformer shown in Fig. 2.

Referring to the drawings and first to Fig. 1, this is a diagram of connections of a circuit feeding alternating current of high intensity through the lead 164 to the electrode holder 2 of the electrode 3 of an electric melting furnace and through it to the body 5 of molten metal. The condition of contact between the electrode and the holder is supervised and controlled continuously, while the furnace is in operation, or while the supply circuit is connected to a useful load, by providing a measuring instrument which may be permanently connected in the circuit and at any time indicates to the operator whether the contact is good or requires improving. To this end two wires 8 and 9, connected to the holder and the electrode at 6 and 7, respectively, lead to the primary winding 11 of the voltage transformer 10. The potential of the secondary transformer winding 12 is transmitted by wires 13 and 141 to the bridge rectifier 22, while on the other side a wire 14 leads to the contact piece 147 of a small controller Whose normal position is marked by the letter A, where the current from contact piece 147 passes through the contact segment 15, connecting wire 16, segment 17, contact piece and wire 21 to the other side of the rectifier '22 and from one side of the rectifier through wires 23, 143, 24 to the voltage coil 25 of the measuring instrument (cross-coil ohmmeter) 26, from the other side through wire 163, contact 27 and wire 28 to the series resistance 29 allowing adjustment to different measuring ranges, and from this resistance through the brush 144 and Wire 31 to the second end of the voltage coil 25 of the measuring instrument.

The electrode 3 is supplied with courrent through wire 164, the primary winding 33 of the current transformer 32, whose secondary winding 34 is connected on one side to the rectifier 42 by way of wire 35, amperemeter 36 and wire 41, while the other winding is connected to the rectifier by the wire 43. From this rectifier the wire 47 leads to one end of the adjustable shunt 48, while a wire 49 connects the rectifier to the brush 50 of this shunt, whose ends are connected on one side through wires 165, 55 to one end of the current, coil 53 of the cross-coil instrument, on the other side through wires 51, 52 to the other end of this coil.

Since the cross-coil ohmmeter directly indicates the ratio of potential and current, the resistance can be read directly on its scales. Adjustable series resistance 29 and adjustable shunt 48 are provided for the purpose of adapting the instrument to various conditions in different installations. Since this ohmmeter merely indicates the ratio of potential and current, the amperage fed to the furnace is immaterial to the result. As long as the intermediate resistance between the furnace electrode and the holder remains constant, its pointer 161 will remain stationary. Whenever the contact should deteriorate by a loosening of the electrode in the holder or from other causes the pointer will move at once, and if the deterioration of contact reaches a certain limit, the pointer will establish contact between the terminals 54 and 59 of a signalling circuit which comprises the current source 58, one terminal of which is directly connected through wire 57 to the relays 56, while the other terminal is connected to the relays through wire 60, closed contacts 59, 54 and wire 166. c

For example, as illustrated in Fig. 2, the relay 56 may be connected to control, through the conductors 67 and 68, a transformer control winding 713 and/or a relay coil 514. An excess voltage which may arise occasionally, is provided for by a switching relay 130 connected in parallel to the voltage coil 25 of the cross-coil instrument by a Wire 131 leading to the regulating brush 144, the other wire 132 being connected between wires 143 and 23 Any excess voltage in the potential coil of the instrument will cause the relay 130 to open the contact 2'7. Further means servin as a protection against excess voltage will be described farther below.

In the case of extremely low voltage it is advisable to ascertain from time to time whether the contacts in the testing circuit are perfect. To this end a rotatable controller is provided with a separate alternating current source feeding a low tension transformer with a resistanee 73 regulating the current output of the secondary winding 72 which is connected by the wire 74, amperemeter 7 5 and wire 76 to the contact piece 155, while on the other side the wires 78 and 79 le'ad to the contact piece 156. Between the wires 74 and 78 a voltmeter 77 is connected for measuring the voltage output.

When the controller is turned into the first testing position B, current will flow from one terminal of the low tension transformer 70 through contact piece 155, segment 82, wire 83, segment 84, contact piece 148, wire 149 and wire 13 to one end of the winding 12 of the voltage transformer 10. The other end of winding 12 i's connected to the source of current by means of the contact piece 156, segment S7, wire 86, segment 85, contact piece 147 and wire 14. Hereby this winding 12 is excited and a voltage is induced in the other winding 11, which, if all the contacts are in good order, is shortcircuited through wire 9, contact 7 of the electrode 3, wire 8 and the contact 6 of the electrode holder 2. The deflection of the pointer of amperemeter 75 at a predetermined deflection or the voltmeter 77 then allows to readily ascertain whether the contacts 7 and 6 at the electrode and its holder are in order. If they should be oxidized; a materially lower current will pass through. In every installation the empirically determined values are ascertained by tests. If the deflection of the amperemeter is found to be too low, the voltage at the regulatin'g resistance 73 is increased until the instruments show that good contact is reestablished.

By setting the small controller to the position C, the other side of the circuit of the voltage coil is then tested. To this end one pole of the voltage transformer is connected with one side or the rectifier 22 by way of contact piece 155, segment 88, wire 89, segment 90, contact piece 150 and Wire 21. The other pole is connected with the other side of the rectifier 22 by way of contact piece 156, segment 93, wire 92, segment 91, contact piece 148, Wire 149 and wire 141.

By applying a predetermined voltage and varying it any deflection of the pointer 161 of the cross-coil instrument 26 will be noticed, while the current coil 53 is fed by the working current through the voltage transformer 32.

The check-up device hereabove described can be used also with advantage for testing any other part of the apparatus.

While Figure 1 illustrates an electrode and electrode holder of a furnace, I wish it to be understood that this is merely one example out of the great number of points of contact in an alternating current installation which can be controlled and protected in the manner here described. As described in my said parent application, Serial No. 495,311, the principle underlying this invention may be applied to both alternating current and direct current systerns.

In contradistinction to ordinary measuring methods, the present invention is not primarily concerned with the-actual magnitude of the ohmic resistance of a contact, the main purpose being to let the operator know when a deterioration has taken place which renders it necessary for him to take preventive action.

Fig. 2 illustrates an adaptation of my invention to the rotection of lever switches. The figure shows a double pole circuit breaker 511. The combination of devices, to be described further below, for testing its contacts and for protecting it comprises, among other instruments, an auxiliary device serving for preventing the testing instruments from being placed under the full working voltage before the circuit-breaker has been thrown in fully. On the other hand, when the breaker is thrown out again, the auxiliary instrument must first be cut out in order that the testing system be not exposed unnecessarily to an excess voltage.

The principle underlying this auxiliary device is embodied in an auxiliary switch inserted in the testing wires leading to the potential coil or the measuring instrument, and the relay for actuating the auxiliary switch is connected to the output side of the circuit breaker. Consequently the relay will throw in the auxiliary switch only after the circuit breaker has been closed and the voltage is available at the output side. A system of well-known locking devices provides for the auxiliary switch to be first opened before the main switch (circuit breaker) is thrown out.

This device can be used in connection with all kinds of switches and for direct and alternating current as explained in my parent application. For simplicity, alternating apparatus is herein illustrated. It is useful more particularly in all cases where the main switches are not readily accessible or hidden to the eye and dif-* ficult to be supervised. In the case of lever switches lacking an automatic actuating device and being arranged for manual operation, the auxiliary switch is also actuated manually. It may, for instance, be thrown in by the bridge of the lever switch, after the lever switch has already passed through part of its path. If the lever switch is opened manually, the auxiliary switch will be opened by means of a spring.

In the combination of a circuit breaker and an auxilia'ry switch of the kind above described, as illustratedin Fig. 2, the current enters through wires 501 and 506. There is a circuit breaker 511 comprising movable contacts 503 and 508, cooperating with input terminals 502 and 507 respectively and output terminals 504 and 509. 513 is the contact (normally closed) and 519 is an auxiliaryswitch (normally open) actuated by the sole noid coil 516 which coil is connected to the output terminal 509 of the circuit breaker. From this terminal the current flows through wire 517 to one end of the solenoid coil 516, while from the other output terminal 504 current passes through Wire 512, contact 513 and Wire 515 to the other end of the solenoid coil 516.

As soon as the circuit breaker reaches its end position, a full network voltage arises between the terminals 504 and 509 and excites the coil 516, causing the auxiliary switch 519 to establish connection between the wires 518 and 207. The rod 539 of the solenoid is suspended by means of a pull spring 526. A catch 527 pivoted to this rod at 528 can move upwardly only while being prevented from turning downwardly by the stop 529. On the solenoid 516 being actuated, the catch 527 meets the arm 530 pivoted at 531 and after having been turned upwardly, moves underneath the arm 530.

Owing to the wires 207 and 518 having been connected with each other, the potential coil 25 of the cross-coil instrument 26 is operatively connected with the input and output terminals of one pole of the circuit breaker, the input terminal by Way of wire 518, contact 519 and wire 207, the output terminal 504 through Wire 205. The two wires 205 and 207 are connected to transformer leads 8 and 9 respectively. The potential coil of the cross-coil instrument is now operatively connected in the circuit, while the current coil 53 (Fig. 1) is operatively connected by means of leads 35 and 43 to the shunt 523 which is connected With the output terminal 504 by wire 505.

If a particularly great deterioration of the circuit breaker contacts sheuld occur, they would at once he cut out by way of Wires 67, 68, the relay coil 514 being energized, leaving contact 513 open and interrupting the circuit of solenoid coil 516, whereby the pull spring 526 is enabled to pull the solenoid back and auxiliary switch 519 is opened. The connection leading to the cross-coil instrument is the first to be interrupted. At this moment the catch 527 meets arm 530 and by turning it reassumes its initial position above arm 530. The turning of this arm causes its other end 532 to close the contact 534, thereby closing a circuit, leading to the trip coil 539 of the circuit breaker, as follows: from one pole of the source of current 537 through wire 538 to one end of the trip coil 539 and from the other end of the same source of current through wire 536, arm 532, contact 534, and wire 535 to the other end of the same trip coil. This causes instantaneous opening of the circuit breaker. The spring 533 returns arm 532 and all th other parts into their initial positions.

Thus the purpose of the auxiliary switchto be thrown in after and thrown out before the main switch (circuit breaker) is attained.

If it is desired to throw out the breaker in the normal manner at the end of the working process, the circuit of the relay coil 514 with its wires 67 and 68 is actuated by means of a separate push-button (not shown) to open the contact 513.

In the same manner as described above, the circuit breaker will in this case be opened also, since a trip coil is actuated.

Fig. 3 illustrates the application of this invention to the protection of circuits comprising high tension oil break switches. Here also an auxiliary switch will be provided which in view of the high tension will best be designed similarly to a high tension oil switch and for the same voltage, however for a lower amperage. This auxiliary high-tension switch is actuated by the auxiliary low-tension switch, whose mode of operation has been described with reference to Fig. 2.

' In the system shown in Fig. 3 the high tension enters through the wires 601, 602, 603 connected to contacts 604, 609 and 639, respectively. The wiring diagram is shown complete for one phase only; it is identical for the other phases.

A voltage transformer 614 is connected to two breaker terminals of the two phases on the output side of the high tension oil switch by means of the breaker terminal 606, wire 613, fuse 615, primary winding 616 of the transformer, fuse 617 and wire 618 leading to the second breaker terminal 611. The secondary winding 619 of this voltage transformer feeds an auxiliary low voltage switch 519 of the same design and arrangement as described with reference to Fig. 2, the numerals which denote similar parts in both figures being the same throughout.

At the moment when the main high tension oil breaker switch is thrown in, the breaker terminals 606, 611 receive the tension whereby the voltage transformer 614 is excited, whose secondary winding 619 feeds the solenoid coil 516 exactly as described with reference to Fig. 2. At the same time the auxiliary low voltage switch 519 is closed whereby the wires 621 and 622 are connected and a low tension source 620 is closed which excites a solenoid coil 623, the current flowing from the current source 620 through wire 621, auxiliary switch 519 and wire 622 to one end of the solenoid coil 623. The other end of the coil is connected through wire 624 to the same current source 620.

The coil 623 on being excited attracts the rod 625 which by means of the movable brush 628 connects the input and output contacts 629 and 627 of the auxiliary high tension switch with each other. Only following this connection the drop of potential arising at the contacts of the main oil switch is transmitted to the testing and protecting device through the output contact 606, wire 634 and high tension fuse 635 to one end of the primary winding 633 of the voltage transformer 6 632.v The input terminal 604 of the main high tension is connected by means of wire 626, one terminal of the auxiliary high tension switch 627, brush 628, the second terminal of the high tension switch 629, wire 630 and fuse 631 to the other end of the primary winding 633.

To the secondary winding 636 of the voltage transformer 632 are connected the wires 13 and 14 which correspond in every respect to the wires 13 and 14 in Fig. 1 and which also feed the voltage to the potential coil of the cross-coil instrument (not shown) exactly as in Fig. 1.

The output wire 607 of the high tension system eX- tends to the primary winding 640 of the current transformer 639. To the secondary winding 642 are connected the wires 35 and 43 which again, as in Fig. 1, lead to the current coil of the cross-coil instrument.

As shown in Fig. 3, whenever the high tension oil switch is" thrown in, the potential and current coils of the cross-coil instrument are thrown in also and their operation is exactly alike to that of the corresponding parts in Fig. l. The operator is thus enabled to obtain at any time during operation a true picture of the actual state of the contacts of the h gh tension switch. If the current is cut out under high overcharge or under direct short circuit, it is possible to ascertain instantly after switching in again, whether the contact has deteriorated. Byreading the instruments the operator is enabled to decide during which opening interval the high tension switch should be dismounted and overhauled.

Here also for every type of oil switches a certain intermediate resistance is ascertained and determined empirically. If by deterioration of the contacts the intermediate resistance of the high tension oil switch should unexpectedly rise very quickly and exceed a predetermined maximum value, then the protective apparatus will release a signal and/ or cut out the switch altogether.

Here also, as in Fig. 2, the auxiliary switch (in this instance a high tension switch), is thrown out first, thereby effecting the throwing out of the entire measuring apparatus and the main high-tension oil switch. To this end the signal sender 66 (see Fig. 1) sends the impulse through wires 67, 68, as already explained with reference to Fig. 2, to the releasing coil 514 of the contact 513. Again the opening of this contact also opens the auxiliary low-tension switch 519 inserted between wires 621 and 622 and the current in the closing coil 623 is interrupted. The auxiliary high-tension switch is thus opened at once.

As explained with reference to Fig. 2, when the rod 539 is pulled back, the rotatable arm 532 closes a circuit 537 which actuates the trip coil of the main high-tension switch.

Exactly as in the case of low tension, if the main high- =tension switch shall be thrown out, an additional push button switch (not shown) is first actuated, which sends an impulse through wires 67, 68 to the coil 514 for the purpose of opening the auxiliary contact 513.

The means here shown and described for locking the auxiliary switch can of course be replaced by any other electrical or mechanical locking devices. g

In certain arrangements it may be advisable to provide an interlocking device with time delay action. This delay could amount to a fraction of second or any necessary longer time. A time delay relay could be advisable, for instance, to avoid any interference with or influence on the instrument 26 due to the capacitance or induction elfect which may be greater in high voltage systems. The delayed switching of the protective device can be accomplished, as mentioned, not only automatically but also by hand.

In some certain cases where no other disturbing effect like capacitance or induction action can be feared, by the switching in of the protective device, we have to consider only that the arrangement never connects the instrument before the switch which has to be supervised iscdmplet'ely in. The same safety rule requires that the supervised switch is never disconnected b'efore the instrilment itself is disconnected;

In Figs. 2 and 4 I have shown the new voltage transformer serving for transforming the low voltages to the secondary winding without allowing any high voltage that may arise, to reach this winding, and serving furdiet for keeping the magnetizing current of'the primary winding within predetermined limits.

The transformer here shown has four limbs 701, 702, 703 and 704. Limb 702 carries the primary winding 633 shown in Fig. 3, while limb 701 which has a very small cross-sectional area and is very thin, carries the secondary winding 636.

A third limb 703 having a large cross-section is formed with an air gap 707. The fourth limb 704 having a large cross-section is formed with a large air gap 714. A core 710 extending into the gap 714 is normally held in a position where it leaves the gap open by the spring 715 acting on the plunger 712 in the closing coil 713 which on being excited, attracts the plunger and causes the core to close the gap. With a normal tension or a fraction of 1 volt the entire flux of magnetic lines of force traverses the small limb 701 because the large limbs with their air gaps at so low a magnetization density offer a far too great magnetic resistance, and consequently a fairly complete voltage transfer is obtained. The dimensions of the small limb 701 should however be so chosen that already at a tension of a few volts a full magnetic saturation is obtained. I thereby provide that on the secondary side the tension can rise only up to a prede termined value, since the saturation does not allow the lines of force to be increased indefinitely in the small limb. At a predetermined voltage the magnetic circuit of the large limb 704 is closed artificially by the solenoid coil 713 driving the core 710 into the gap 714. This coil is excited by the signalling contact 66 through wires 67, 68 of the cross-coil instrument, and this instrument allows varying of the voltage at which the solenoid is excited. It may for instance be excited at the moment where a great deterioration of the intermediate resistance arises and besides the optical and acoustical signal also means for reducing the current intensity or for throwing out the circuit breakers are actuated.

On the gap 714 being closed by the core 710, the greatest part of the magnetic lines of force will find a new passage offering a much lower magnetic resistance. I thereby succeed in keeping the magnetizing current of the primary winding 633 within predetermined limits, thereby preventing this winding from being destroyed if the primary tension rises strongly.

Since the iron of the small limb 701 is already saturated, the entire increase of the magnetic flux will for the greater part only take place in the limb 704.

According to the ratio of the cross-sectional areas and the magnetic resistances of the two limbs I can obtain that almost no further increase of the lines of force occurs in the small limb and no further rise of the secondary induced tension takes place. At the moment where the magnetic core enters the air gap, the tension may 'drop on the secondary side. Therefore a relay such as provided for instance in the telephone bell lines will maintain the signal until it is interrupted manually by the operator.

By correspondingly choosing the ratios of the limbs, practically any ratio of the voltage normally to be tested and the maximumexcess voltage on the primary side can be obtained and the secondary voltage can be kept as low as desired.

In the practical operation of the protective 'device a retardation may occur in the action of the solenoid whereby the tension may rise further before the whole system has been cut out. In such a'case the magnetization of the large limbs 703 and 704 will increase and a 8. considerable part of these lines of force pass through the core 710. When the tension has risen sufficiently, the magnetic lines of force will increase at this point to such an extent that the core 710 is attracted automati= cally. In this manner the voltage transformer will achieve automatically, althohgh with a little delay, the effect normally to be achieved by the solenoid.

It might even happen that the gap 714 in the large limb 704 is not closed at all. In that case the second large limb 703 will step in and after a predetermined voltage (magnetization) has been reached, the greater part of the magnetic lines of force will start to pass through the air gap 707 of this limb. In this way a similar effect is otbained as by closing the gap in the large limb 704, and in this case as well the secondary winding 636 is not exposed to an unduly high tension.

In exceptional cases the intermediate resistance in a contact may rise momentarily to a very high value, even as high as several thousand volts, before the main high tension switch is cut out, and then the magnetization current of the primary winding will also rise considerably. By correspondingly calculating the iron conditions, this magnetization current can be so adjusted that the high tension fuse shown at 635 and 631 in Fig. 3 will blow out when a predetermined voltage and herewith also a predetermined magnetization current is exceeded.

Apart from this protection for the measuring devices, they are also protected by the excess voltage cut-out in Fig. 1.

Obviously the core closing the air gap may have the form of a wedge and it may have any desired cross-section (quadrangular, conical, cylindrical, etc.) and it may be as wide as or less wide than, the limb. The two air gaps 7-14 and 707 might also be provided at a single limb. We may also provide that simultaneously with the closing of gap 714 an air gap is formed in the small limb 701 which for this purpose is made in two parts. In each case the magnetic flux, when rising, is ofiered a new, artificially produced path of lower magnetic resistance.

Obviously also the number of limbs and the relative arrangement of the limbs to be opened and closed may be varied. Thus another limb with an air gap closed by a magnetic wedge, or another limb with a permanently open gap may be provided. The secondary limb may carry more than one winding, which may serve different purposes. Similar designs may also be used in the current transformers for the protection of the instrument.

The offering of a new path of lower magnetic resistance for the magnetic lines of force by closing an air gap may also be applied with advantage to the measuring instruments themselves in order to render them safe against the action of excess voltages.

We wish it to be understood that we do not desire to be limited to the exact details disclosed in the specification and drawings, for obvious modifications will occur to persons skilled in the art.

We claim:

1. In an electrical distribution system apparatus responsive to operative state of an electrical element thereof comprising an electro responsive measuring instrument, means for connecting said element to said distribution system and an interlock for connecting said electro responsive instrument subsequent to the connection of said element in said electrical distributio'nsystem and for disconnect'ing said elect'ro responsive device prior to disconnection of said element, the interlock comprising a normally open switch having contacts adapted to be closed in series with said electro-responsive device, an energizing winding for said switch adapted to be connected in said electrical distribution system and a normally closed switch in series with said winding having a trip coil energized by said electro-r-esponsive device for disconnecting said device in response to excessive actuation thereof.

2. In an electrical distribution system having a :power switch adapted to connect a source of power thereto,

the combination of power switch input and output terminals, and switch contact testing apparatus comprising in combination first conductors connected to such power switch input and output terminals and second conductors connected to the output terminals of such power switch a voltage-responsive device having connections to said first conductors, a normally open contactor having con tacts interposed in said connections and having an actuating winding connected to said second conductors for energization upon closing of said power switch.

3. Apparatus as in claim 2 wherein the actuating winding for the normally open contactor has interposed in series therewith a pair of normally closed contacts having a trip coil adapted to be energized by said electro-responsive device upon energization thereof exceeding a predetermined value.

10 4. Apparatus as in claim 3 wherein the power switch is provided with a tripping circuit for opening said switch and the normally open contactor is provided with auxiliary contacts for closing said tripping circuit upon movement of said normally open contactor toward the opening position.

References Cited in the file of this patent UNITED STATES PATENTS 1,188,593 Wild June 27, 1916 1,199,936 Simon Oct. 3, 1916 1,822,866 Baber Sept. 8, 1931 1,825,978 Philbrick Oct. 6, 1931 1,906,817 Seeley May 2, 1933 2,254,707 Nakayama Sept. 2, 1941 2,345,420 Palme Mar. 28, 1944 

